Malloc() and free() stopping me from passing structs by value in C

Question

I'm working on a project and I keep getting segmentation faults, and the values of the structs aren't being passed. Figuring out why is driving my crazy. I've tried figuring out the problem with simpler programs, and I think I've found the problem, but I'm not sure how to fix it.

The problem is that when I "malloc" a struct, THEN pass by value, the value is lost. Adding "free" later produces a segmentation fault. I'm not trying to access a value from before the "malloc()" or after the "free()", so I'm confused as to why this is happening.

Here's a simple model of the problem:

#include <stdlib.h>
#include <stdio.h>

    struct structexample
    {
        int element;
    };

    void initStruct(struct structexample * teststruct, int * number)
    {
        teststruct = malloc(sizeof(struct structexample)); 
        teststruct->element = 10;
        printf("teststruct element is %d in initStruct\n", teststruct->element);
        *number = 5;
    }

    void printtest(struct structexample * teststruct, int * number)
    {
        printf("teststruct element is %d in printtest\n", teststruct->element);
        printf("Number is %d\n", *number);
        free(teststruct);
    }

int main()
{
    int number;
    struct structexample teststruct;
    initStruct(&teststruct, &number);
    printtest(&teststruct, &number);
    printf("teststruct element is %d in main()", teststruct.element);
    return 0;
}

This produces:

teststruct element is 10 in initStruct
teststruct element is -7967792 in printtest
Number is 5
Segmentation fault

I compile the program with "gcc -Wall -pedantic -ansi" and get no errors or warnings there.

When I comment out "malloc" and "free" it correctly produces:

teststruct element is 10 in initStruct
teststruct element is 10 in printtest
Number is 5

If I only comment out "free" but leave "malloc" in, that fixes the segmentation fault, but the values of the structs are still incorrect. In this simple program I don't really need "malloc()" and "free()", but I do need them in my larger project. If I can make them work in this simpler program, then I think I can fix the larger one. I can't find a similar problem on Google unfortunately.

Answer 1

void initStruct(struct structexample * teststruct, int * number)
{
    teststruct = malloc(sizeof(struct structexample)); 
    teststruct->element = 10;
    printf("teststruct element is %d in initStruct\n", teststruct->element);
    *number = 5;
}

You pass this function a value, the address of a structure, which it totally ignores. It allocates with malloc, but does nothing with the address it got back. It doesn't return it. It doesn't put it anywhere other code can get it. So you've just leaked this object.

void printtest(struct structexample * teststruct, int * number)
{
    printf("teststruct element is %d in printtest\n", teststruct->element);
    printf("Number is %d\n", *number);
    free(teststruct);
}

So printtest passes teststruct to free, so it should be passed a value returned from malloc.

struct structexample teststruct;
initStruct(&teststruct, &number);
printtest(&teststruct, &number);

But you pass printtest the address of teststruct, which is allocated on the stack. That can't be right.

Are you expecting initStruct to somehow move teststruct? It surely can't do that.